Firefighters who worked at Ground Zero are 19% more likely to have cancer than those who were not exposed to the site, a new study finds.

“This is not an epidemic, but an increased risk,” cautioned lead author Dr. David Prezant, the chief medical officer of the New York City Fire Department (FDNY), in a conference call with reporters on Thursday.

The new study, published Thursday in the journal Lancet, is being described as the first meaningful research on the question of whether exposure to toxic dust at the World Trade Center site increased responders’ cancer risk.

Although firefighters, police officers and other Ground Zero workers have long pointed to anecdotal evidence and said that their exposure has undeniably caused cancer, researchers say most solid tumors take decades to grow, so such a link could take up to 40 years to prove.

Still, some lawmakers and advocates for Ground Zero responders hope the new findings will help persuade the administrators of a 9/11 health program enacted by Congress in 2010 to provide site workers with medical care and compensation to include payment for cancer treatment. The $4.3 billion program does not currently cover cancer care.

Earlier this year, the National Institute for Occupational Safety and Health, the agency that periodically reviews available data and determines which conditions should be covered by the 9/11 health program, ruled that there was no connection between dust exposure and cancer.

Some experts reviewing the new research also said that the modest excess risk of cancer found among the nearly 9,000 Ground Zero firefighters included in the study was not statistically significant. The study identified 263 cases of cancer among Ground Zero-exposed firefighters (when 238 would have been expected in the general population), compared with 153 cases in non-exposed firefighters (161 cases would have been expected in the general population).

The study’s author, however, argued that the findings are significant — and made more notable because they are unexpected. The increase in cancer risk identified by the study occurred in in the first seven years after the World Trade Center attack, which was not anticipated, the study’s author said. “We excluded cancers that might have been diagnosed early [that may have existed before the attack] … and we still see a 19% increase,” Prezant told CNN.

Another expert who spoke with CNN suggested that unique characteristics of Ground Zero dust could have contributed to accelerated disease in workers at the site:

Researchers have reported the presence of hundreds of compounds in Ground Zero dust, among them known carcinogens. Potential cancer-causing agents such as asbestos that coated the Trade Center buildings’ lower columns, and benzene, a component of jet fuel that caused uncontrollable fires when planes barreled into the twin towers, have been a cancer concern for researchers.

So have the high volume of particulates and gases inhaled by firefighters, said Prezant.

“Those particulates are not just dust, they are dust coated with the same chemicals that were in the air in terms of the gases, sometimes, actually, getting deeper into the lung or better penetrating into the blood circulation because they’re carried on a particle,” said Prezant.

Despite the increase in overall cancer cases found in the study, there weren’t enough cases to conclude anything about the risks of individual cancers, when the data were broken down by disease type. Interestingly, though, the researchers found 58% fewer cases of lung cancer among Ground Zero firefighters than would have been expected in the general population.

Prezant acknowledges that the findings are just a starting point and that longer-term research is needed. “We’ve just begun to understand what’s happening after the World Trade Center,” said Prezant. “We may find that some of our conclusions change over time, get stronger or change entirely.”

The key question for many Ground Zero responders is whether the findings will convince officials to include payments for cancer in the 9/11 health program. Given that the study was limited to firefighters and that the findings are still preliminary, they are not expected to do so.

Dr. James M. Melius, director of the New York State Laborers’ Health Fund and one of the leading advocates for ground zero workers suffering health problems, said that even though the cancer research on firefighters was inconclusive, it showed enough possibility of a risk that U.S. officials should consider adding cancers to a list of conditions covered by a multibillion dollar health aid bill passed by Congress last year.

Doing so would qualify exposed people for sizeable compensation payments. “Are we going to wait until we have definitive evidence, which could be 20 or 30 years? Are we going to say, decades from now, ‘Yeah, you did get cancer because of the World Trade Center, and we should have helped you out back then?'” he said. “It’s limited information. It isn’t a perfect study … It still provides compelling evidence that we should be providing at least health care for these people.”

A separate study published Thursday in the Lancet examined 2003-09 New York City mortality data for about 42,000 people — rescue workers and civilians — who were potentially exposed to Ground Zero dust and found no increase in death risk. The data suggested that the study group had a 43% lower risk of death from any cause than New Yorkers in general. They were also less likely to have suffered fatal respiratory disease or blood cancer.

These findings were attributed to the “healthy worker” effect, however: because the terrorist attacks happened in a business district, the people who were present at the site were presumably employed, a group that is typically healthier than the general population.